Conglomerate, Racemate, and Achiral Crystals of Polymetallic Europium(III) Compounds of Bis- or Tris-β-diketonate Ligands and Circularly Polarized Luminescence Study

This work reports (a) conglomerate and racemic crystal structures of [(Δ,Δ,Δ,Δ,Δ,Δ)- or/and (Λ,Λ,Λ,Λ,Λ,Λ)-EuIII6(TTP)8(OH2)6Na4]n coordination polymers, (b) racemic crystal structures of (Δ,Δ,Δ,Δ)-/(Λ,Λ,Λ,Λ)-EuIII4(TTP)4(bipy)4(MEK)2(OH2)2 tetrahedral clusters, and (c) the achiral crystal structure of the [EuIII2(BTP)4(OH2)2Na2]n coordination polymer (where BTP = dianionic bis-β-diketonate, TTP = trianionic tris-β-diketonate, and bipy = 2,2′-bipyridine). The screw coordination arrangement of the TTP ligand has led to the formation of homoconfigurational racemic EuIII products. The conglomerate crystallization of [EuIII6(TTP)8(OH2)6Na4]n appears to be caused by the presence of the sodium, Na+ counterions, and interactions between oxygen atoms and the trifluoromethyl unit of the TTP ligand and Na+ ions. All the EuIII compounds exhibit characteristic red luminescence (5D0 → 7FJ, J = 0–4) in solution or in the solid crystalline state. Circularly polarized luminescence (CPL) was observed in the chiral EuIII6(TTP)8(OH2)6Na4]n species, displaying a |glum| value in the range of 0.15 to 0.68 at the 5D0 → 7F1 emission band. Subtle changes of the [EuIII6(TTP)8(OH2)6Na4]n structure which may be due to selection of twinned crystals or crystals that do not correspond to a perfect spontaneous resolution, are considered to be responsible for the variation in the observed CPL values.


■ INTRODUCTION
Induction and control of chirality are the central topics of organic, polymer, and inorganic chemistry. Chiral coordination complexes have been extensively developed and acknowledged for their key role in stereoselective synthesis, which has been rewarded the 2001 Chemistry Nobel Prize given to Noyori, Knowles, and Sharpless. 1−5 Chiral polymers are widely used in high-performance liquid chromatography (HPLC) for the separation of chiral substances. 6−8 Recently, chiral substances have also been in the spotlight for their promising properties as chiral light emitters. Circularly polarized luminescence (CPL) has attracted considerable attention for applications in optoelectronics, OLEDS, security tags, or luminescent probes. 9−16 Chiral coordination substances based on d-group and f-group elements, such as Pt(III), Ru(II), Au(I), Eu(III), Tb(III), and others have been widely investigated. 17−33 Recently, metal clusters have also been reported for their CPL capability. 34−36 Several approaches are considered to systematically induce chirality in the coordination complexes: intrinsic chirality of the metal clusters, 37,38 the use of chiral ligands, 23,39,40 or the generation of an asymmetric arrangement around the metal center using an achiral ligand. The synthesis of many CPL-active enantiopure complexes relies on the second approach. The latter method, less predictable, often leads to a racemic mixture, which can be quite arduous to separate, requiring specialized techniques such as chiral HPLC or chiral ion-pairing. Particularly for species such as lanthanide complexes, which possess inherent kinetic lability and generally have a rather fluxional coordination sphere, 41 the spontaneous resolution of enantiomers, also known as conglomerates, remains to be the most challenging but promising route to obtain CPL materials. In the last decade, spontaneous resolution of conglomerate lanthanide(III) (Ln III ) coordination complexes and coordination polymers has been reported based on multi-dentate ligands. 42−49 Intermolecular interactions observed in the Ln III crystal structures predominantly govern the conglomerate crystallization process. Despite several examples of solid-state circular dichroism, CD, 42,44−47 the CPL of conglomerate crystals has only been reported by Zhu et al. in a chiral molecular organic framework (CMOF). 49 Due to significant advantages of low costs, high efficiency, and easy scale-up, the conglomerate approach of CPL lanthanide complexes is desired to adapt to the solution-phase processability.
We here report chiral Eu(III) coordination polymers based on conglomerate crystallization and their CPL activity in solid powders and the solution phase. New Eu III coordination polymers are composed of a tris-β-diketonate ligand (1,3,5tris(3-trifluoromethyl-3-oxopropanoyl) benzene, H 3 TTP).

C r y s t a l S t r u c t u r e D e s c r i p t i o n s . T h e [Eu III
6 (TTP) 8 (OH 2 ) 6 Na 4 ] n was obtained from the reaction between Eu III chloride hexahydrate and H 3 (TTP) in a 3:4 stoichiometry ratio in the presence of sodium hydroxide. Single crystals were successfully grown from slow diffusion of diethyl ether into a solution containing the sample in acetone. Several attempts (12 times) in crystallization resulted mostly in conglomerate crystal structures of homoconfigurational (Δ,Δ,Δ,Δ,Δ,Δ)-or (Λ,Λ,Λ,Λ,Λ,Λ)-[Eu III 6 (TTP) 8 ] coordination polymers (Figure 1a, Figure S1, and Table 1 for three sets of crystallographic data). The chiral conglomerate has a Flack parameter in the range of 0.220(4) to 0.480(4), confirming highly enantiopure crystal structures. A (Δ,Δ,Δ,Δ,Δ,Δ)-[Eu III 6 (TTP) 8 ] monomer unit exhibits a distorted capped square antiprism (CSAP) geometry in their coordination sphere where nine vertices are saturated with eight βdiketonate oxygen atoms and an oxygen atom of a coordinating water molecule (Figure 2a,b) with an average Eu−Eu distance of 10.092 Å, which seems short enough for an Eu−Eu exciton interaction. 50,51 The Eu−O(β-diketonate oxygen) and Eu− O(OH 2 ) distances are in the range of 2.378(7)−2.493(7) Å and 2.450(6)−2.548(7) Å, respectively. The total charges of all Eu III and shared Na + ions in a [Eu III 6 (TTP) 8 ] unit are −6 and 5, respectively. Other sodium ions bridge two Eu cites of adjunctive [Eu III 6 (TTP) 8 ] cages through interactions with oxygen and CF 3 units of the TTP ligand and water molecules ( Figure 2d, Table S1) in a compressed octahedral geometry, assembling [Eu III 6 (TTP) 8 ] units into a 2D coordination polymer, which spreads on the a−b plane (Figure 2f). A 2D polymeric structure was also reported by Yang et al. 52 The  6 Na 4 ] n . Only a [Eu III 6 (TTP) 8 ] unit is shown. (b) Racemic crystal structures of (Δ,Δ,Δ,Δ)-/ (Λ,Λ,Λ,Λ)-Eu III 4 (TTP) 4 (bipy) 4 (MEK) 2 (OH 2 ) 2 . (c) Crystal structure of [Eu III 2 (BTP) 4 (OH 2 ) 2 Na 2 ] n . Only a [Eu III 2 (BTP) 4 ] unit is shown. Solvents, hydrogen atoms, and/or Na + counterions have been omitted for clarity. Eu III −Na + distances are in the range of 3.405(6)−3.568(4) Å (Table S1). Eu 1 , Eu 2 , Eu 5 , and Eu 6 ions of a [Eu III 6 (TTP) 8 ] unit are connected to Eu 5 , Eu 2 , Eu 1 , and Eu 6 ions of another unit by Na + ions, respectively. Conversely, Eu 3 and Eu 4 ions are not connected to other Eu ions. The remaining Na + ions are observed between the 2D nanosheet, which are tethered to a [Eu III 6 (TTP) 8 ] unit by a CF−Na + interaction ( Figure 2e). We also obtained achiral [Eu III 6 (TTP) 8 (OH 2 ) 6 Na 4 ] n crystals where (Λ,Λ,Λ,Λ,Λ,Λ)-and (Δ,Δ,Δ,Δ,Δ,Δ)-[Eu III 6 (TTP) 8 ] hexacore cages form a 1:1 racemic crystal. Unfortunately, we could not obtain a fully refined analysis because of crystallographic disorder in this racemate. However, this still gives us precious information on the real structure of such crystals as shown in Figure 3 (see also Figure S2a,b and Table S2). The trigonal antiprism geometry is preserved with average Eu−Eu distances of 10.104 Å (Figure 3a,b). A partial 1D polymer framework is clearly observed with bridging Na + ions between Eu 2 atoms of (Λ,Λ,Λ,Λ,Λ,Λ)-[Eu III 6 (TTP) 8 ] units with Eu 3 atoms of (Δ,Δ,Δ,Δ,Δ,Δ)-[Eu III 6 (TTP) 8 ] units through  interactions with the oxygen atom and CF 3 group of the TTP ligand in a similar fashion compared with the conglomerate structure ( Figure S2b,c). The emergence of polymorphic architectures reminds the authors the importance of kinetic control of crystal nucleation and growth. Both types of [Eu III 6 (TTP) 8 ] units seem to co-exist in the solution phase; between which, interconversion is slower than the nucleation and growth. 53 We further conducted reactions between the Eu III chloride hexahydrate and H 3 (TTP) in a 1:1 stoichiometry ratio in basic conditions at room temperature to prepare the precursor powders of Eu III 4 (TTP) 4 (sol) n (sol = OH 2 or CH 3 OH), which were not characterized. We tried to grow crystals from dimethoxylethane (DME) and a hexane solvent system and obtained a 1:1 racemic crystal of (Δ,Δ,Δ,Δ)-and (Λ,Λ,Λ,Λ)-Eu 4 (TTP) 4 (DME) 4 (sol) 4 . Due to high disorder of the DME molecules, the crystal structures are poorly resolved and are shown in Figure S3. Despite the insufficient structural characterization of the precursor, we could reliably use it for the sequential reaction with 2,2′-bipyridine (bipy) in a 1:4 stoichiometry ratio to form other 1:1 racemic crystals of (Δ,Δ,Δ,Δ)-and (Λ,Λ,Λ,Λ)-Eu III 4 (TTP) 4 (bipy) 4 (MEK) n , which crystallized in a tetragonal system with the space group P4̅ c2 (Figure 1b, Figure 4, and Table S4). The racemic crystal structures exhibit a Flack parameter of 0.008(2). They possess a nearly T-symmetrical tetrahedral architecture where four nona-coordinated Eu III ions occupy the apexes of the tetrahedron and four trianionic TTP ligands make up the four triangular faces (Figure 4a). The average Eu−Eu distance is 10.062 Å. Each nona-coordinated Eu III ion exhibits a distorted CSAP geometry where nine vertices are occupied with two nitrogen atoms of the bipyridine ligand, six oxygen atoms of three β-diketonate moieties of THP ligands, and an oxygen atom of the coordinating solvent molecule (MEK) ( Figure  4b). CF−F interactions between THP ligands ( Figure S6) appear to stabilize the final T symmetrical complexes in the solid-state and solution ( Figures S4d and S5d).
Another reference substance [Eu III 2 (BTP) 4 (OH 2 ) 2 Na 2 ] n was produced from the reaction between Eu III chloride hexahydrate and H 2 (BTP) in a 1:2 stoichiometry ratio in basic conditions, which was successfully crystallized in a triclinic system with the space group P1̅ from a solvent diffusion technique (acetone and chloroform; Figure 1c, Figure 5, and Table S4). A Eu III 2 (BTP) 4 (OH 2 ) 2 Na 2 monomer consists of two Eu III ions and quadruple strands of the dianionic BTP ligand (Figure 5a; the Eu III −Eu III distance is 7.3304(6) Å). The sodium ions act as counterions to neutralize the doubly charged complexes of [Eu III 2 (BTP) 4 ]. Two oxygen atoms of BTP ligands connect a Na + and a Eu III ion together (Figure 5c). The Eu III −Na + distance is 3.791(2) Å. A Na + ion is bridged to an adjacent Na + ion by two water molecules, generating a linear coordination polymer (Figure 5d; Na + −Na + distance = 3.507 (3)  Photoluminescence and CPL Properties. Upon excitation at the ligand absorption band (λ = 360 nm), E u I I I 4 ( T T P ) 4 ( b i p y ) 4 ( M E K ) 2 ( O H 2 ) 2 a n d [Eu III 6 (TTP) 8 (OH 2 ) 6 Na 4 ] n exhibit characteristics of red Eu III luminescence ( 5 D 0 → 7 F J , J = 0−4) in solution and in crushed crystalline powder. Their solution emission profiles are depicted in Figure 6. They display a single narrow line in the 5 D 0 → 7 F 0 emission band and several crystal-field splitting lines in 5 D 0 → 7 F 1 emission bands. Intense photoluminescence was observed in hypersensitive 5 D 0 → 7 F 2 transition, which is associated with the non-centrosymmetric nona-coordinated Eu III cores. 54 Due to the different Eu III coordination environm e n t s a s r e v e a l e d i n t h e c r y s t a l s t r u c t u r e s , Eu III 4 (TTP) 4 (bipy) 4 (MEK) 2 (OH 2 ) 2 exhibits different spectral line patterns in 5 D 0 → 7 F 2 emission bands.
Circularly polarized luminescence (CPL) occurs when an emitting species displays intrinsic chirality or stands in a chiral environment. The CPL activity is characterized by the dissymmetry factor (g lum ), defined as g lum = 2 (I L − I R )/(I L + I R ), where I L and I R refer to the left and right circularly polarized intensity, respectively. We used the lab-designed CPL system with excitation illumination and emission correction at same sides of samples. Automatic correction of linearly polarized component minimizes the artifact for reliable CPL profiles of powder samples. In the solid-state CPL study, two pieces of crystals were randomly selected among samples in a vessel containing mainly homo-chiral crystals, C 1 and C 2 ,   which were characterized as [(Δ,Δ,Δ,Δ,Δ,Δ)-and (Λ,Λ,Λ,Λ,Λ,Λ)-Eu III 6 (TTP) 8 (OH 2 ) 6 Na 4 ] n , respectively, after the X-ray crystallographic analysis. As the crystals gradually undergo degradation under air, CPL analysis was performed with the powder sample deposited on quartz plates. As shown in Figure 7, almost complete mirror CPL images were obtained for C 1 and C 2 , respectively. Clear CPL profiles are shown for the specific Eu III transitions, namely, 5 D 0 → 7 F 1 at approximately 596 nm and 5 D 0 → 7 F 4 at approximately 614 nm. The highest g lum values, that is, −0.29 for C 1 and +0.1 for C 2 , were evaluated at the magnetic dipole transition (λ = 595 nm). This is no surprise as this transition satisfies the magnetic-dipole selection rule, ΔJ = 0, ±1 (except 0 ↔ 0), and often shows particularly large circular polarization. 40,55,56 Several justifications can explain the difference in maximum g values evaluated between the two samples: it may be attributed to the different levels of degradation of each crystal after being extracted from the crystallization solution, or alternatively, it may be due to the selection of twinned crystals or crystals that do not correspond to a perfect spontaneous resolution (crystals that are intermediate between conglomerate and racemate, containing, e.g., 90% a given enantiomer). These last explanations are also consistent with the reported values of the Flack parameter, which are considerably higher than 0. In order to assess the chiral structure in the solution phase, the powders were dissolved in acetone and CPL was monitored as shown in Figure 7b. Interestingly, the initial solution phase sample showed CPL profiles that are almost identical to those in the crystalline phase with a similar intensity ratio and opposite phases at 5 D 0 → 7 F 1 and 5 D 0 → 7 F 4 transitions.  [(Δ,Δ,Δ,Δ,Δ,Δ)-or (Λ,Λ,Λ,Λ,Λ,Λ)-Eu III 6 (TTP) 8 (OH 2 ) 6 Na 4 ] n luminescence and CPL spectra of the re-dissolved crystals in acetone. Solution from C 1 is in red, and solution from C 2 is in blue. All measurements were performed at 298 K. 570−630 nm (left); 640−720 nm (right).
Although we are not fully confident with the reliability, the magnetic dipole transition of 5 D 0 → 7 F 1 indicated g lum values of −0.15 and +0.68 for C 1 and C 2 , respectively.
The CPL capability of the acetone solution of C 1 was further analyzed after one week, demonstrating some retention of notable CPL activity (Figure 8). The stored solution showed an identical emission profile (Figure 8) with the freshly prepared solution (Figure 7b). The CPL signals at 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 transitions were also essentially same as of the original state. The absolute value of the dissymmetry factor, g lum , at the 5 D 0 → 7 F 1 changes from −0.15 to −0.1, whereas 5 D 0 → 7 F3 and 5 D 0 → 7 F 4 transitions disappear totally. It is hypothesized that the [(Δ,Δ,Δ,Δ,Δ,Δ)-/(Λ,Λ,Λ,Λ,Λ,Λ)-Eu III 6 (TTP) 8 (OH 2 ) 6 Na 4 ] n goes dissolved with a minor modification of the coordination structure, preserving the CPL activity. The slow rearrangement/decomposition or randomization of the self-isolated chiral structure over time could be responsible for the slow decay on CPL activity (Figures 7b and 8). The multi-nuclear Eu III chiral cage seems to be kinetically stable and considerably suppresses the racemization in the solution phase.
CPL Measurements. The CPL spectra were measured using the lab-designed CPL system consisting of an excitation laser at 375 nm, Hinds PEM-90 photoelastic modulator with a frequency of 50KHz, Hamamatsu H7732 photomultiplier tube with a signal amplifier, polarizing prism, and Shimadzu monochromator (10 cm, single grating). The system was previously calibrated, and detailed information has been discussed in a previous manuscript (J.